Absent response to cold calorics localizes coma to the brainstem. MRI is superior to CT for posterior fossa infarcts; CT is often first to rule out bleed.
Abolished caloric responses in a comatose patient localizes to the brainstem, and MRI (DWI) is more sensitive than CT for posterior fossa infarcts.
A 76-year-old man from a skilled nursing home presents to you with an acute change in mental status. He is usually alert and oriented to person, place, and time, and he was without symptoms or concerns prior to today's presentation. He was last seen approximately 12 hours ago and was his normal self at that time. Now he is nonverbal and non-responsive to painful stimuli or cold water in the external auditory meatus.
Which of the following is a likely cause of this finding, and what is the superior diagnostic test?
Answer Options:
A. Cerebral hemorrhage: noncontrast computed tomography (CT)
B. Cerebral tumor: contrast computed tomography (CT)
C. Brainstem lesion, contrast computed tomography (CT)
D. Brainstem lesion, magnetic resonance imaging (MRI)
Profound unresponsiveness plus failure to respond to cold caloric testing (oculovestibular reflex) suggests dysfunction of the brainstem arousal system and/or brainstem reflex pathways rather than an isolated supratentorial process. On boards, this is a classic “localize the coma” question: if key brainstem reflexes are absent, think brainstem lesion (including posterior circulation ischemic stroke).
For imaging, MRI with diffusion-weighted imaging is superior to CT for detecting acute ischemia in the posterior fossa/brainstem, where CT performance is limited by beam-hardening artifact from the skull base. This fits contemporary stroke imaging principles in AHA/ASA ischemic stroke guidance (2019 update to 2018 framework) and ACR Appropriateness Criteria: CT is often the initial ED test for speed and hemorrhage exclusion, but MRI is the more sensitive/superior diagnostic study for posterior circulation infarction when rapidly available and not delaying time-critical care.
| Option | What It Tests / Implies | Why It’s Wrong Here |
|---|---|---|
| Cerebral hemorrhage: noncontrast computed tomography (CT) | Recognizing hemorrhagic stroke and CT-first hemorrhage evaluation | A hemorrhage can cause coma, but the absent caloric response points more specifically to brainstem dysfunction; also the question asks “likely cause” given the described reflex finding. |
| Cerebral tumor: contrast computed tomography (CT) | Subacute/chronic intracranial mass workup with contrast imaging | Tumors rarely cause sudden profound coma without preceding symptoms/signs of increased ICP or focal deficits. |
| Brainstem lesion, contrast computed tomography (CT) | Thinking “brainstem lesion” but choosing CT w/ contrast | Contrast CT is not the preferred test for acute posterior circulation ischemia; noncontrast CT is for hemorrhage screening, while MRI (DWI) is best for ischemia detection in posterior fossa. |
| Brainstem lesion, magnetic resonance imaging (MRI) | Correct localization + best sensitivity study | MRI (DWI) is most sensitive for acute brainstem/posterior fossa infarct and avoids CT posterior fossa artifact; aligns with stroke imaging principles (AHA/ASA 2019 update; ACR criteria). |
Absent oculocephalic/oculovestibular (caloric) responses in coma localize to the brainstem, and MRI-DWI best detects posterior fossa ischemia.
Localize coma using brainstem reflexes (including oculovestibular testing).
Choose the most sensitive imaging modality for suspected posterior fossa/brainstem ischemic stroke.
The stem uses a “localizing physical exam” (no response to cold calorics) to force you away from generic stroke algorithms and toward brainstem pathology, then asks for the superior test (MRI) rather than the most common rapid ED first-line test (NCCT).
A comatose patient has absent oculovestibular reflexes bilaterally and no gag reflex. Which anatomic structure is most implicated?
A 68-year-old has acute vertigo, dysarthria, and limb ataxia. Noncontrast head CT is unrevealing. Best next imaging to detect an acute posterior circulation infarct?
In suspected acute ischemic stroke triage, what is the primary purpose of emergent noncontrast head CT in most EDs?
A comatose patient has unequal pupils and abnormal oculocephalic reflexes. This combination most strongly suggests:
Which clinical finding most suggests basilar artery occlusion requiring emergent stroke evaluation?
How would your imaging choice and urgency differ between (1) suspected basilar artery occlusion with coma and abnormal brainstem reflexes vs (2) toxic-metabolic coma with preserved pupillary and caloric responses?
Q1: Why does absent response to cold calorics point to the brainstem?
A: The oculovestibular reflex arc traverses vestibular nuclei and ocular motor nuclei in the pons/medulla; ABEM-style questions use this to localize coma to the brainstem.
Q2: If MRI is “superior,” why do we often start with noncontrast CT?
A: The ABEM expects you to know NCCT is rapid and best to exclude hemorrhage immediately (AHA/ASA), while MRI-DWI is more sensitive for ischemia—especially in the posterior fossa—when available without delaying care.
Q3: What’s the main CT limitation in posterior fossa strokes?
A: Beam-hardening artifact from the skull base reduces sensitivity for brainstem/cerebellar ischemia, so MRI-DWI is favored for detection.
Q4: Does the “12 hours last known well” change the imaging principle tested here?
A: It mainly tests recognition of posterior circulation localization and MRI sensitivity; reperfusion eligibility is a separate layer and depends on protocols, vascular imaging, and advanced imaging selection.
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